1. Field of the Invention
This invention relates to optical inspection and measuring systems, and more specifically, to an optical system incorporating a resonator to enhance detection of optical path differences in structures under inspection.
2. Description of the Related Art
Optical structures, such as fiber optic cable, beam splitters, combiners and couplers, as well as active optical components are presently increasing in use and complexity. Optical systems are desirable for secure and high speed processing and communications, so manufacture of optical structures is increasing due to market demands.
Recently, planar optical structures have been developed that incorporate optical devices, fiber interconnects and connectors within a thick layer mounted on a substrate. Planar optical structures enable the manufacture of high-density optical processing and telecommunication systems.
In the manufacturing process, it is necessary to inspect and measure planar optical structures, to determine whether or not they are manufactured to the tolerances demanded by functional requirements and to make necessary adjustments in the manufacturing process to avoid manufacturing defective components.
Typically, these planar optical structures are multi-layer structures comprising regions of differing refractive indices. For example, an optical waveguide may be formed by embedding a channel of rectangular cross section within a surrounding material, wherein the channel has a higher refractive index than the surrounding material. In order to inspect the dimensions of the above-described waveguide, it is necessary to detect the horizontal position of both sides of the waveguide and the vertical thickness of the waveguide. The same measurements can be abstracted to more complex optical structures.
The overall path difference between the cladding and core of an optical structure is due to the relatively thin region of slightly differing refractive indices corresponding to the thickness of the core. Therefore, high sensitivity to small differences in the optical path length (and hence the information containing the dimensions of the optical structures) is desired.
Therefore, it would be desirable to provide an inspection method and apparatus having an enhanced sensitivity to differences in optical path length within a device under inspection.
The foregoing objectives are achieved in an optical measurement and inspection method and apparatus having enhanced path length measurement sensitivity. The inspection apparatus includes an optical illumination system for producing a beam for illuminating a device under inspection, a detector for detecting light reflected from the device under inspection and resonator positioned between the illumination subsystem and the device whereby the sensitivity to phase of light detected with the detector is improved due to multiple reflections occurring within the resonator.
The foregoing and other objects, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings.